This invention relates to an optical finger image generating apparatus. In particular, the invention relates to an apparatus for generating an image from an optical scan of a finger supported on a platen.
A number of techniques are known for generating an image which is equivalent to an inked fingerprint. In many finger imaging devices, a fingerprint image is stored as electronic data and evaluated by a computer to identify a person or to verify the identity of the person. Such systems are useful, for example, in security systems which restrict access to locations or equipment. The capabilities of such identification systems and other systems relying on a finger imaging apparatus are limited by the difficulty of providing a finger imaging apparatus which is both convenient to use and which provides a clear, consistent finger image. Features of the finger such as bifurcations and endings of ridges and valleys are especially important for fingerprint identifications.
Systems have been developed which employ a layer of liquid to improve the imaging of ridge and valley portions of the finger. The use of a liquid layer, however, tends to filter out important details in the finger image. In addition, systems involving a liquid layer are messier and more inconvenient than systems using a dry finger. Accordingly, a system capable of providing a finger image with a high level of contrast between ridge and valley portions as well as a high level of detail is necessary for accurate fingerprint identification.
Because the finger surface may have cracks or other features which interfere with proper imaging of the ridges and valleys of the finger, it is often desirable to filter the finger image in order to eliminate such features while keeping the minutae which are used in fingerprint identification. The use of a finger imaging apparatus with a liquid layer filters out unwanted features, but the level of filtering cannot be controlled and even important minutae of the finger may be filtered out of the image. In systems employing a dry platen, filtering of the finger image may be performed electronically, but such filtering requires additional costly electronic equipment.
Several prior art finger imaging devices make use of frustrated total internal reflection (frustrated TIR). In such systems, light is incident on a transparent platen at an angle below the critical angle. In a region where nothing is in contact with the platen, e.g. at a valley portion of a finger in contact with the platen, the incident light is reflected from the platen. A photodetector is oriented to image light experiencing TIR from the platen. Where something such as a ridge portion of the finger is in contact with the platen, the TIR does not take place, and the ridge portion is imaged as a dark region by the photodetector. However, residual material on the platen can also frustrate TIR and result in a dark portion of the finger image. This is especially problematic in the case of traces of finger oil left on the platen by previously imaged fingers. The finger oil frustrates TIR and results in detection of a ghost image of the earlier finger. This reflected "noise" interferes with the generation of an accurate finger image.
It is an object of the present invention to provide an optical finger imaging apparatus. In particular, it is an object of the present invention to provide such an apparatus which generates a finger image from an optical scan of a finger supported on a platen.
In particular, it is an object of the invention to provide an apparatus which generates a finger image with a high contrast between ridge and valley portions.
A further object of the present invention is to provide an apparatus which generates a finger image with a high resolution.
An additional object of the present invention is to provide a finger imaging apparatus which enables a controlled level of optical filtering of the finger image.
A further object of the present invention is to provide a finger imaging apparatus which overcomes the problem of noise from platen residue.
These and other objects of the present invention will be apparent from the drawings and detailed descriptions herein.